Plasma doping systems are known and used for forming shallow junctions in semiconductor wafers and for other applications requiring high current with relatively low energy ions. In plasma doping systems, a semiconductor wafer is placed on a conductive platen, which functions as a cathode and is located in a plasma doping chamber. An ionizable doping gas is introduced into the chamber, and a voltage pulse is applied between the platen and an anode or the chamber walls, causing formation of a plasma containing ions of the dopant gas. The plasma has a plasma sheath in the vicinity of the wafer. The applied pulse causes ions in the plasma to be accelerated across the plasma sheath and to be implanted into the wafer. The depth of implantation is related to the voltage applied between the wafer and the anode. Very low implant energies can be achieved. Examples of such plasma doping systems are described in U.S. Pat. No. 5,354,381 to Sheng, U.S. Pat. No. 6,020,592 to Liebert et al., and U.S. Pat. No. 6,182,604 to Goeckner. In the above described plasma doping systems, the applied voltage pulse generates a plasma and accelerates positive ions from the plasma toward the wafer. In other types of plasma systems, a continuous plasma is produced, for example, by inductively-coupled RF power from an antenna located internal or external to the plasma doping chamber. The antenna is connected to an RF power supply. At intervals, voltage pulses are applied between the platen and the anode, causing ions in the plasma to be accelerated toward the wafer.
Dopant gas species used for plasma implantation may decompose or dissociate during the implant process into atomic or molecular fragments which may be deposited on the surface of the wafer. Atomic or molecular fragments that result from dissociation of dopant gas molecules are referred to herein as “neutral particles.” Examples of dopant gas species which dissociate during the implant process include AsH3, PH3, BF3, and B2H6. For example, arsine gas AsH3 may dissociate into As, AsH and AsH2, which may be deposited on the surface of the wafer being implanted. These deposited surface layers can cause a number of problems, including dose non-repeatability, poor dose uniformity and dose measurement problems. In particular, the neutral particles that form the deposited surface layers are not measured by the dose measurement system. Further, the depth profile of the dopant is altered by the deposited surface layer itself and by its effect on implanted ions. In addition, the deposited surface layers can cause contamination of other equipment, such as annealers, when the wafers are subsequently processed in such equipment.
Accordingly, there is a need for providing a profile adjustment solution in plasma doping applications to meet both concentration and junction depth requirements.